sheep: fix gcc various warnings

sheep.c:133:6: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
...
cluster/shepherd.c:278:7: warning: variable ‘removed’ set but not used [-Wunused-but-set-variable]
...
cluster.c:279:8: warning: ignoring return value of ‘strtol’, declared with attribute warn_unused_result [-Wunused-result]

Fix uninitialized_var() and use it to keep GCC happy.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>